1. Field of the Invention
This invention relates to an apparatus for forming a sample solution such as blood or urine into a flat sheathed flow, irradiating the flat flow of the sample solution with strobe light to obtain a still picture, and employing image processing to perform analysis such as classification and enumeration of particle components contained in the sample solution. More particularly, the invention relates to a particle image analyzing apparatus adapted to constantly monitor an image pick-up area and irradiate the particles in the sample solution flow with strobe light when they reach the image pick-up area, thereby making it possible to acquire an image of the particle components in an efficient manner even if the sample has a low particle content.
2. Description of the Prior Art
When the particle components contained in a sample of blood or urine taken from a living body are to be examined, the conventional practice is to prepare a sample by smearing the sample on a glass slide and observing the slide under a microscope to classify or enumerate the particle components. However, the conventional method is laborious and lacking in accuracy. Accordingly, in order to reduce the labor involved in examination and to improve accuracy, automatic analyzing apparatus have been developed, a specific example of which is disclosed in the specifications of Japanese Patent Application Laid-Open (KOKAI) No. 57-500995 and U.S. Pat. No. 4,338,024. The disclosed apparatus uses a strobe light to irradiate a sample solution sheathed in sheathing liquid and formed into a very flat flow, acquires a still picture by means of a video camera and subjects the picture to image processing to classify and/or enumerate material components contained in the sample. An automatic urinalysis apparatus to which this art is applied is already available on the market.
FIG. 15 is a block diagram showing the construction of the conventional apparatus of the kind described above. The apparatus includes an image processor 200 which produces a strobe triggering signal at fixed intervals to fire a strobe 202 at a regular time interval. The emitted strobe light is made to irradiate a sample solution, which flows through a flat passageway 208 in a flow cell 206, by an optical system 204 comprising lenses and other elements. The sample solution, which travels in a direction perpendicular to the plane of the drawing, flows in a broad width vertically of the drawing but in a small width horizontally thereof within the flat passageway 208. Light which has passed through the flow cell 206 has its image formed on a CCD light-receiving surface 214 of a video camera 212 by an optical system 210. The video camera 212 produces a video signal output in synch with a generator-lock signal from the image processor 200, and the video signal is subjected to image processing within the image processor 200. The strobe 202 is powered by a strobe power supply 216.
FIG. 16 is an enlarged view of a portion of the sample-solution flow as seen from the image pick-up side. Numeral 220 denotes the flat flow of the sample solution, in which particle components 222 are contained.
The sample solution flows from left to right in FIG. 17, which is a perspective view of the image pick-up area. Numeral 224 denotes a portion of the sample-solution flow whose image is picked up by the video camera 212. In FIG. 16, numeral 226 denotes a portion of the sample-solution flow imaged just prior to portion 224, and numeral 228 denotes a portion of the sample-solution flow to be imaged following portion 224.
Ordinarily, the image pick-up operation performed by a video camera is such that one frame has a duration of 1/30 of a second. If the sample is irradiated with light every 1/30 of a second and the total measurement time is 45 seconds, then 1350 image frames can be acquired. However, if the sample solution has a low particle-component content, particle images will not necessarily appear in all of the frames. For example, assume that the sample is blood and that leukocytes are to be analyzed. In order to perform leukocyte analysis, the blood sample used is one in which the red blood cells have been subjected to hemolytic disruption and the leukocytes have been stained.
Now assume that a sample obtained by subjecting blood having a leukocyte content of 5000 cells/.mu.l to the aforesaid pretreatment and finally diluted by ten times, namely a blood sample having a leukocyte content of 500 cells/.mu.l , is passed through a flow cell and analyzed. Assume also that the image pick-up area is a square each of whose sides has a length of 150 .mu.m, and that the thickness of the flat sheathed flow is 8 .mu.m (in which case the volume of the image pick-up area will be 150 .mu.m.times.150 .mu.m.times.8 .mu.m=1.8.times.10.sup.-4 .mu.l).
As mentioned above, FIG. 17 is a perspective view of the image pick-up area. Numeral 230 denotes a leukocyte. When the leukocyte count per imaged frame is determined under the aforementioned conditions, the figure obtained is 500 cells/.mu.l.times.1.8.times.10.sup.-4 .mu.l=0.09 cells. In other words, only one leukocyte appears in 11 imaged frames. This means that even if 1350 frames are obtained, as mentioned above, the number of frames in which leukocytes appear will be 1/11 of the total number of frames, or about 120 frames, according to simple calculation. In order to increase the number of frames in which leukocytes appear, three methods are conceivable: (a) lower the dilution rate of the sample (i.e., raise the concentration); (b) shorten the image pick-up cycle; and (c) enlarge the image pick-up area (volume).
Method (a) is disadvantageous in that the hemolysis of the red blood cells may be inadequate and the amount of blood necessary may need to be increased. With method (b), a special video camera capable of performing photography at 100 or more frames per second can be used, but such a camera is very expensive. Furthermore, it is necessary to raise the speed of image processing as well. Another problem is that since the irradiation cycle of the strobe is shortened, the quantity of light emitted can become erratic and the life time of the strobe may be shortened. When the image pick-up area is enlarged (i.e., when the magnification is reduced) according to method (c), the size of the cell image becomes smaller in relative terms. In addition, when the thickness of the float sheathed flow is increased, there is an increase in the number of cells which do not appear in focus, and the end result is a decline in cell-image resolution.
Thus, cell images cannot be obtained efficiently using the conventional method in which strobe light is emitted every 1/30 of a second to effect imaging.
In order to obtain cell images more efficiently, an effective expedient would be to irradiate a cell with the strobe light to obtain its image only when the cell arrives at the image pick-up area, and suspend photography when there are no cells in the image pick-up area. Examples of devices that could achieve this are an electrical-resistance-type detector comprising micropores and a pair of electrodes, and an optical detector comprising light-emitting and light-receiving elements. With such an arrangement, the presence of a cell would be detected by the detector. However, there is no assurance that a cell which has passed by the detector will always pass through the image pick-up area. The sample solution flows in a broad transverse spread in the proximity of the image pick-up area, and hence there are cases where cells miss the image pick-up area during their passage through the flow cell. In addition, the length of time from the moment a cell passes the detector until it reaches the image pick-up area tends to differ depending upon a variety of conditions. For this reason, cells cannot always be photographed at a correct timing.